The present invention relates to a sensor assembly with a first, second and third layer of flexible material. The layers form a pressure-sensitive sensor having at least two sensor cells.
Such a sensor is known from DE 10 2015 120 368 B3, for example.
Generic multi-layer sensors are characterized by having at least one layer of a pressure-sensitive material that changes its electrical volume resistance at the point of loading under a local mechanical loading. Electrodes above and below the pressure-sensitive layer detect the change in resistance and thus make it possible to determine the location and the strength of a pressure load on the sensor assembly. In this way, flexible, flat sensors can be created that can be used in safety mats or input devices.
The basic principle of such tactile sensors is described in GB 2 115 555 A. GB 2 115 555 A discloses a tactile sensor in which the pressure-sensitive layer is formed in the form of an elastic mat. The mat can be a textile woven or felt material that is permeated with a carbon or metallic additive, making the mat conductive overall. The fibers of the mat, which have been made conductive, act together under a pressure load on the mat, so that a contact resistance through the mat changes in the place of the pressure load. According to GB 2 115 555 A, strip-shaped electrodes are arranged matrix-like above and below the mat to measure the variable volume resistance and coupled to an electrical circuit. The electrodes are, for example, metallic foils or metal-added silicones. Via a voltage applied to an upper and a lower electrode, the electrical resistance in the overlap area of the respective electrodes can be determined and thus the contact resistance of the elastic mat at this location. The measured volume resistance in turn allows conclusions to be drawn about the respective pressure load at this point. By determining in succession the resistance between an upper and a lower electrode, a pressure distribution over the elastic mat can be determined.
The measuring principle is therefore based on determining the variable volume resistance of the pressure-sensitive layer in order to determine a pressure distribution. Decisive for the properties of the sensor is therefore the nature of the pressure-sensitive, electrically conductive material and its ability to change its volume resistance. In case of a very thin layer, a change in the contact resistance due to a compressive load may only be marginal and thus large-area electrodes are necessary in order to detect a change in the volume resistance. The size of a sensor cell, which is determined by the overlapping areas of the electrodes, is therefore directly dependent on the layer thickness of the pressure-sensitive layer, thereby limiting unfavorably the resolution of a sensor with many sensor cells.
Another disadvantage is that very closely spaced sensor cells can influence each other, in that the variable volume resistance in the area of one cell affects the adjacent cell. In order to minimize this effect, adjacent sensor cells must therefore be spaced further apart, resulting in a dead zone between the sensor cells, in which the pressure load cannot be effectively detected.
DE 10 2007 022 871 A1 addresses these problems and proposes to at least partially interrupt the pressure-sensitive, conductive layer in order to electrically decouple two adjacent cells from each other. In a first embodiment, DE 10 2007 022 871 A1 discloses therefore a sensor, in which the pressure-sensitive, conductive layer is divided into individual pads, which are each placed at intersection points of two electrodes and are also spaced apart by air or another medium which has a greater electrical resistance. Thereby, the individual cells are electrically completely decoupled from each other, yet still areas, in which the decoupling takes place, form sections in which a pressure load cannot be effectively detected.
In a second embodiment, DE 10 2007 022 871 A1 alternatively discloses that the pressure-sensitive, conductive layer can be designed to be a continuous layer, however, individual areas of the pressure-sensitive, conductive layer are designed such, for example by cutting or milling pieces, so that webs are formed between two adjacent sensor cells which electrically decouple two adjacent sensor cells from one another. The pressure-sensitive, conductive layer thus remains continuous, but requires appropriate processing to form the webs in the form of high-impedance bridges.
Each of the variants disclosed in DE 10 2007 022 871 A1 therefore requires that the pressure-sensitive, conductive layer must be processed and adapted, either by dividing it into individual components or by converting areas into high-resistance bridges. Both make the overall design of the sensor expensive and time-consuming. In addition, restrictions with regard to resolution or dead areas in which the sensor cannot effectively detect a pressure load must be tolerated.